Abstract 394: Bacterial Serum Opacity Factor Rescues HDL Functionality in SR-B1 -/- Mice

Abstract

Although plasma HDL-C levels negatively correlate with atherosclerotic cardiovascular disease (ACVD), attempts to reduce ACVD risk by raising plasma HDL have disappointed. Thus, hypotheses about salutary HDL effects have shifted from higher-is-better to function-is-more-important. The SR-B1 -/- mouse is an extreme model of HDL dsyfunctionality; compared to WT mice, SR-B1 -/- mice have higher plasma HDL levels and an HDL surface that is free cholesterol (FC)-rich (60 vs. 15 mol%). This would be expected to increase HDL-FC bioavailability to cytotoxic levels. HDL dysfunctionality among SR-B1 -/- mice is associated with multiple metabolic abnormalities—impaired cell membrane structure and function and atherosusceptibility, despite having high plasma HDL-C levels; moreover, female SR-B1 -/- mice are infertile. Liver-specific SR-B1 expression in SR-B1 -/- mice normalizes HDL size and FC content. Thus, the SR-B1 -/- mouse phenotype is due to lack of hepatic clearance of lipids from dysfunctional HDL. Serum opacity factor (SOF) is a bacterial protein that catalyzes the quantitative disproportionation of HDL into a cholesteryl ester-rich micro emulsion (CERM), neo HDL, and lipid-free apo AI. The CERM contains apo E and all HDL-CE. Injection of SOF (4 μg) into WT mice lowers plasma cholesterol by diverting the CERM to hepatic LDLR. Thus, we began testing whether adeno-viral delivery of SOF (AAV SOF ) to SR-B1 -/- mice rescues HDL functionality. A plasmid encoding the SOF gene was synthesized; SOF DNA was isolated by restriction enzyme digestion and cloned into a pAAV-TBG-mcs plasmid that was submitted to the PENN Vector Lab for virus production and isolation. AAV GFP plasmid (UPENN) was used as control. Good SOF production and secretion was confirmed by transfection of Huh7 hepatocytes. AAV SOF injection into SR-B1 -/- mice induced constitutive plasma SOF activity and reduced HDL-C levels to nil. Superposition of high plasma HDL levels and a high mol% FC in SR-B1 -/- is expected to increase HDL-FC bioavailablity that contributes to whole-body FC-toxicity and the observed metabolic abnormalities. Future tests will determine whether ablation of dysfunctional HDL in SR-B1 -/- mice rescues their pathological phenotype, especially atherosclerosis.